Cartridge History

A Brief History of the Various Cartridges for the AR Platform

5.56×45mm NATO

The 5.56×45mm NATO (official NATO nomenclature 5.56 NATO) is an intermediate cartridge developed in the United States and originally chambered in the M16 rifle. Under STANAG 4172, it is a standard cartridge for NATO forces as well as many non-NATO countries. It is derived from, but is not identical to, the .223 Remington cartridge.

Development and History

In the 1950s, the 7.62×51mm NATO rifle cartridge (physically interchangeable with, but not identical to, the .308 Winchester rifle cartridge) was selected to replace the .30-06 Springfield as the standard NATO rifle cartridge. At the time of selection, there had been criticism that the 7.62×51mm NATO was too powerful for lightweight modern service rifles, causing excessive recoil, and that as a result it did not allow for sufficient automatic rate of fire from hand-held weapons in modern combat.

The British had extensive evidence with their own experiments into an intermediate cartridge since 1945 and were on the point of adopting a .280 inch (7 mm) cartridge when the selection of the 7.62×51mm NATO was made. The FN company had also been involved in the development of the .280 round including developing a version of the FN FAL in .280. The concerns about recoil and effectiveness were effectively overruled by the US within NATO, and the other NATO nations accepted that standardization was more important at the time than selection of the ideal cartridge.(The EM-2, Rifle No.9 Mk1 or "Janson rifle", was an experimental British assault rifle briefly adopted by British forces in 1951, but the decision was overturned very shortly thereafter by Winston Churchill's incoming government in an effort to secure NATO standardization of small arms and ammunition in the face of American intransigence.) However, while the 7.62×51mm NATO round became NATO standard, the US was already engaged in research of their own, which ultimately led to the 5.56×45mm NATO cartridge.

During the late 1950s, ArmaLite and other U.S. firearm designers started their individual Small Caliber/High Velocity (SCHV) assault rifle experiments using the commercial .222 Remington cartridge. When it became clear that there was not enough powder capacity to meet U.S. Continental Army Command's (CONARC) velocity and penetration requirements, ArmaLite contacted Remington to create a similar cartridge with a longer case body and shorter neck. This became the .222 Remington Special. At the same time, Springfield Armory's Earle Harvey had Remington create an even longer cartridge case then known as the .224 Springfield. Springfield was forced to drop out of the CONARC competition, and thus the .224 Springfield was later released as a commercial sporting cartridge known as the .222 Remington Magnum. To prevent confusion among all of the competing .222 cartridge designations, the .222 Remington Special was renamed the .223 Remington. With the U.S. military adoption of the ArmaLite M16 rifle in 1963, the .223 Remington was standardized as the 5.56×45mm NATO. As a commercial sporting cartridge the .223 Remington was introduced in 1964.

The 5.56×45mm cartridge, along with the M16 rifle, were initially adopted by U.S. infantry forces as interim solutions to address the weight and control issues experienced with the 7.62×51mm round and M14 rifle. In the late 1950s, the Special Purpose Individual Weapon program sought to create flechette rounds to allow troops to fire sabot-type projectiles to give a short flight time and flat trajectory with a muzzle velocity of 1,200 meters per second (3,900 ft/s) to 1,500 meters per second (4,900 ft/s). At those speeds, factors like range, wind drift, and target movement would no longer affect performance. Several manufacturers produced varying weapons designs, including traditional wooden, bullpup, "space age," and even multi-barrel designs with drum magazines. All used similar ammunition firing a 1.8 mm diameter dart with a plastic "puller" sabot filling the case mouth. While the flechette ammo had excellent armor penetration, there were doubts about their terminal effectiveness against unprotected targets. Conventional cased ammunition was more accurate and the sabots were expensive to produce. The SPIW never created a weapons system that was combat effective, so the M16 was retained, and the 5.56 mm round was kept as the standard U.S. infantry rifle cartridge.

In a series of mock-combat situations testing in the early 1960s with the M16, M14 and AK-47, the Army found that the M16's small size and light weight allowed it to be brought to bear much more quickly. Their final conclusion was that an 8-man team equipped with the M16 would have the same fire-power as a current 11-man team armed with the M14. U.S. troops were able to carry more than twice as much 5.56×45mm NATO ammunition as 7.62×51mm NATO for the same weight, which would allow them a better advantage against a typical NVA unit armed with AK-47, AKM or Type 56 assault rifles. (*AK-47/AKM magazines are much heavier than M14 and M16 magazines)

In 1977, NATO members signed an agreement to select a second, smaller caliber cartridge to replace the 7.62×51mm NATO cartridge. Of the cartridges tendered, the 5.56×45mm NATO was successful, but not the 55 gr M193 round used by the U.S. at that time. The wounds produced by the M193 round were so devastating that many consider it to be inhumane. Instead, the Belgian 62 gr SS109 round was chosen for standardization. The SS109 used a heavier bullet with a steel tip and had a lower muzzle velocity for better long-range performance, specifically to meet a requirement that the bullet be able to penetrate through one side of a steel helmet at 600 meters. This requirement made the SS109 (M855) round less capable of fragmentation than the M193 and was considered more humane.

The 5.56×45mm NATO inspired an international tendency towards relatively small-sized, lightweight, high-velocity military service cartridges that produce relatively low bolt thrust and free recoil impulse, favoring lightweight arms design and automatic fire accuracy. Similar intermediate cartridges were developed and introduced by the Soviet Union in 1974 (5.45×39mm) and by the People's Republic of China in 1987 (5.8×42mm).

6.5mm Grendel

6.5mm Grendel

The 6.5mm Grendel (6.5×39mm) is an intermediate cartridge designed by Arne Brennan, Bill Alexander and Janne Pohjoispää as a low recoil, high accuracy, 200-800 yard cartridge specifically for the AR-15 platform. It is an improved variation of the 6.5mm PPC. Since its introduction it has proven to be a versatile design and is now expanding out into other platforms including bolt-action rifles and the Kalashnikov system.

The name "6.5mm Grendel" was a trademark owned by Alexander Arms until it was legally released to allow the cartridge to become SAAMI standardized.

The case head diameter of the Grendel is the same as that of the parent case the .220 Russian, the 7.62×39mm, and PPC cases. This is larger than the 5.56×45mm NATO, thereby necessitating the use of a non-standard AR-15 bolt. The increased case diameter results in a small reduction in the capacity of standard size M16/AR-15 magazines. A Grendel magazine with the same dimensions as a STANAG 30-round 5.56 magazine will hold 26 rounds of 6.5mm ammunition.

6.8mm Remington SPC

6.8mm Remington SPC

The 6.8 mm Remington Special Purpose Cartridge (aka 6.8 SPC, 6.8 SPC II & 6.8×43mm) is a rifle cartridge that was developed by Remington Arms in collaboration with members of the U.S. Army Marksmanship Unit, United States Special Operations Command to possibly replace the 5.56 NATO cartridge in a Short Barreled Rifle(SBR)/Carbine.

Based upon the .30 Remington cartridge, it is midway between the 5.56×45mm NATO and 7.62×51mm NATO in bore diameter and muzzle energy. It uses the same diameter bullet (not usually the same weight) as the venerable .270 Winchester hunting cartridge.

Development and History

The 6.8mm SPC cartridge was designed to address the deficiencies of the terminal performance of the 5.56×45mm NATO cartridge currently in service with the U.S. Armed Forces. The cartridge was the result of the Enhanced Rifle Cartridge program. The 6.8 SPC (6.8×43mm) was initially developed by MSG Steve Holland and Chris Murray, a United States Army Marksmanship Unit gunsmith, to offer superior downrange lethality over the 5.56 NATO/.223 Remington in an M16 pattern service rifle with minimal loss of magazine capacity and a negligible increase in recoil. The goal was to create a cartridge that would bridge the gap between 5.56 mm and 7.62×51mm NATO.

The program started the design by using a .30 Remington case, which was modified in length to fit into magazines that would be accommodated by the magazine wells of the M16 family of rifles and carbines that are currently in service with the U.S. Armed Forces.

In tests, it was determined that a 6.5 mm caliber projectile had the best accuracy and penetration, with historical data going back for decades of US Army exterior and terminal ballistic testing, but a 7 mm projectile had the best terminal performance. Further tests showed that a 6.8 mm caliber projectile was a compromise between the two, providing accuracy, reliability and terminal performance up to 500 meters. The combination of the cartridge case, powder load, and projectile easily outperformed the 7.62×39mm Soviet cartridge, with the new cartridge proving to be about 61 m/s (200 ft/s) faster. The resulting cartridge was named the 6.8 Remington Special Purpose Cartridge due to the size of its projectiles and the fact that it was based on the .30 Remington case.

In general, adapting an AR-style rifle to the new cartridge only requires the replacement of the barrel, bolt, magazine & muzzle device (if applicable) of the 5.56 mm-chambered rifle; but to further streamline and simplify the conversion process many parts manufacturers sell complete upper receiver assemblies chambered for 6.8 SPC alongside their conversion kits focusing on the key individual parts. While a complete 6.8 SPC assembly is a somewhat more expensive route, the conversion of an existing 5.56 mm/.223 rifle to 6.8 SPC using a complete upper assembly takes less than a minute on an AR platform rifle without the need for specialized tools or skills. In contrast, when swapping out the individual component parts, a significant level of gunsmithing experience, special tools, and time are generally required to detach the barrel from the rifle's upper receiver and the gas system, and conversely those same needs are required for the reassembly of the upper receiver with the new 6.8 SPC barrel. Also, there is the issue of having to readjust the sights if a new barrel is placed on an existing upper receiver.

The 6.8 mm Remington SPC was designed to perform better in short barreled CQB rifles after diminished performance from the 5.56 NATO when the M16A2 was changed from the rifle configuration to the current M4 carbine. The 6.8 SPC delivers 44% more energy than the 5.56 mm NATO (M4 configuration) at 100–300 meters (330–980 ft). The 6.8 mm SPC is not the ballistic equal of the 7.62×51mm NATO cartridge, but it has less recoil, has been said to be more controllable in rapid fire, and lighter, allowing operators to carry more ammunition than would otherwise be possible with the larger caliber round. The 6.8 mm generates around 2,385 J (1,759 ft·lbf) of muzzle energy with a 7.5-gram (115 gr) bullet. In comparison, the 5.56×45mm round (which the 6.8 is designed to replace) generates around 1,796 J (1,325 ft·lbf) with a 4.0 g (62 gr) bullet, giving the 6.8 mm a terminal ballistic advantage over the 5.56 mm of 588 J (434 ft·lbf). One of the enigmatic features of this cartridge is it being designed for a short barrel carbine length rifle that the standard rifle length is (usually 41 cm (16 in)). The round only gains about 7.6–10.7 m/s (25–35 ft/s) for every 25 mm of barrel length past the standard 410-millimetre (16 in) barrel (all else being equal) up to barrel's length around 560–610 mm (22–24 in) with no gain/loss in accuracy. It also does very well in rifles with less than 410 mm (16 in) barrels. In recent developments (the period 2008-2012) the performance of the 6.8 SPC has been increased by approximately 61 to 91 m/s (200 to 300 ft/s) by the work of one ammunition manufacturer Silver State Armory LLC (SSA) and a few custom rifle builders using/designing the correct chamber and barrel specifications.

300 AAC Blackout (7.62×35mm)

7.62×35mm 300 AAC Blackout

300 AAC Blackout, SAAMI short name 300 BLK, also known as 7.62×35mm is a rifle cartridge developed in the United States by Advanced Armament Corporation (AAC) for use in the M4 carbine. Its purpose is to achieve ballistics similar to the 7.62×39mm Soviet cartridge in an AR-15 platform while using standard AR-15 magazines at their normal capacity.

Development and History

While 5.56×45mm NATO has enjoyed widespread acceptance in military circles, the nature of the missions encountered by some special operations groups often demand a round that not only provides better performance than that available in the high-energy standard velocity rounds, but one that can offer subsonic performance greater than the current standard 9mm Luger round.

In an effort to satisfy this need the 300 AAC Blackout was created by Advanced Armament Corporation in cooperation with Remington Defense, under the direction of AAC's Research and Development Director Robert Silvers and with the support of the company's founder, Kevin Brittingham.

The project's goals were:

Create a reliable compact .30-cal solution for AR platform

Use existing inventory magazines while retaining their full capacity

Create the optimal platform for sound and flash suppressed fire

Create compatible supersonic ammo that matches 7.62×39mm ballistics

Provide the ability to penetrate barriers with high-mass projectiles

Provide all capabilities in a shorter, lightweight, durable, and low recoiling package

Meeting these goals allowed the development team to negate many of the perceived drawbacks inherent to other large caliber cartridges when used in the M4 platform. Colt Firearms and other arms makers had previously chambered AR-pattern rifles and carbines in various .30 caliber rounds but several issues were encountered. In the case of the 7.62×39mm, its relatively severe case angle caused feeding issues unless specially modified AK-47 magazines were used, and even then results were less than outstanding. Modified bolts were also needed owing to its larger case head diameter. Rounds such as the 6.8 SPC and 6.5 Grendel had similar part-interchangeability issues but did allow for the use of the standard M4/M16 30-round magazine albeit with a reduced capacity.

Wildcat cartridges such as the .300 Whisper series addressed these issues but their widespread use in single shot handguns along with the lack of an industry standard cartridge dimension meant that a great number of the popular loads on both the supersonic and subsonic end of the spectrum were less than ideal in the AR pattern weapons. Many of these rounds required an excessively long overall cartridge length that would prohibit feeding in a STANAG magazine while using powder charges that were not compatible with the pressure requirements of the M4 carbine. This was particularly noticeable when using subsonic ammunition in conjunction with a suppressor as short stroking and excessive fouling would occur similar to that which was seen in the earliest variants of the M16 in Vietnam.

By keeping the M4/M16 platform in mind as the primary host during load development the designers were able to work up a host of cartridges that would satisfy not only the ballistic requirements set forth but also ensure mechanical reliability with the fewest changes to the weapon itself, with only a simple barrel change being necessary for a complete conversion.

300 AAC BLACKOUT was approved by SAAMI on January 17, 2011. On October 23, 2011, SSG Daniel Horner of the USAMU used 300 AAC Blackout to win his 4th USPSA Multi-Gun National Championship.